The present invention relates to a swash plate type compressor, and more particularly to improvement in a shoe structure of a swash plate type compressor.
Heretofore, air conditioning systems of the type used, for instance, in automobiles, have employed a swash plate type compressor. This compressor has a cylinder block having a plurality of bores extending parallel to the longitudinal axis of the block, a swash plate rotated by a rotary shaft extending through the cylinder block, pistons slidably fitted in the cylinder bores, and shoes interposed between the pistons and the sliding contact surface of the swash plate through which the pistons are reciprocated by the rotary motion of the swash plate. Rotation of the rotary shaft causes sliding contact rotation of the swash plate, whereby the pistons are reciprocated to compress the refrigerant in the cylinders.
In such a swash plate type compressor, the sliding contact surface of the shoe in contact with the swash plate is acted upon by high load and the slip speed between the shoe and the swash plate is high. As a result, under conditions where the supply of lubricating oil is difficult, such as when the compressor is being started, seizure tends to take place on the sliding contact surface of the shoe, particularly at the middle of the sliding contact surface. The primary reasons why seizure tends to take place on the middle of the sliding contact surface of the shoe are as follows:
(A) Generally, in a swash plate type compressor, it has been common practice to mix lubricating oil in the refrigerant for preventing seizure. This allows the lubricating oil in the refrigerant to be present between the swash plate and the shoe. If, however, the amount of lubricating oil in the refrigerant is increased beyond a certain level, the sliding movement becomes smoother but, on the other hand, the influence of the lubricating oil detracts from the heat exchanging ability of the refrigerant and hinders the circulation of the refrigerant, resulting in a substantial decrease in cooling capability. Therefore, the smaller the amount of lubricating oil mixed in the refrigerant, the greater will be the cooling capability of the system. Thus, the current trend is to reduce the amount of lubricating oil mixed in the refrigerant, thereby increasing the tendency to seize.
(B) During the operation of the compressor, the sliding movement between the swash plate and the shoe causes the shoe temperature to reach a considerably high level of about 200.degree. C. Since the heat of the shoe must be dissipated from the peripheral portions of the shoe which more frequently contact the refrigerant and lubricating oil, the temperature of the middle of the shoe is much higher than the temperature of the peripheral portions of the shoe, with the result that the middle of the shoe tends to bulge and project above the peripheral portion of the shoe due to thermal expansion. Thus, the load tends to be concentrated at the middle of the sliding contact surface of the shoe, increasing the tendency of the shoe to seize at the middle thereof.
(C) During the operation of the compressor, a film of the lubricating oil from the refrigerant is present between the swash plate and the shoe to ensure smooth operation of the compressor. When the compressor is stopped, however, gravity forces out this film of lubricating oil so that there is little or no oil between the swash plate and the shoe. Thus, if the compressor is started after being stopped for a long time, there will initially be a lack of lubricating oil between the swash plate and the shoe, resulting in the sliding contact surface of the shoe tending to seize.